1 of 2 more photos » There was significant seismic activity at the mine, an official from the Utah Department of Natural Resources told CNN.

"An accident occurred during the rescue effort at the Crandall Canyon mine at 6:30 MDT," the Mine Safety and Health Administration said in a statement.

"At this time, it is believed the accident was caused by a 'bump.' We are in the process of conducting a headcount to ensure that everyone is accounted for."

A seismic "bump" is an increase in seismic activity in the area.

At least six ambulances and two helicopters rushed to the site and transported the injured to a nearby hospital.

There "may have been another collapse at the mine" according to officials at Castleview Hospital in Price, Utah. Jeff Manley, CEO of the hospital, said he is expecting up to 11 injured, at least one in very serious condition.

Medics inside the ambulances were seen doing chest compressions. Watch an interview with a hospital spokesman »

"We are in an emergency situation," Bob Murray, president and CEO of mine co-owner Murray Energy Group Corp., told CNN.

Earlier, officials said rescue efforts have been going disappointingly slow, but rescuers have had some encouraging news.

Samples taken from the third hole bored into the mine found that the air had a 16.8 percent level of oxygen, Richard Stickler, head of the U.S. Mine Safety and Health Administration told reporters Thursday.

Murray said video taken from a camera lowered into the hole showed a large, open cavity.

"If the men did go to this area that is open, the air is there, the water is there, everything is there to sustain them indefinitely until we get to them," Murray said.

But he stressed that rescuers do not know if the miners were in that area.

Rescuers are still more than 1,000 feet from reaching the section where the men were believed to be working, Murray said.

Seismic movement Wednesday night loosened rubble that covered the "continuous miner" machine -- a powerful plow that chews up coal and shovels it into carts following behind -- halting work for a while, Murray said. Another shake delayed work later Thursday morning, he said.

"I'm so sorry, ladies and gentlemen, to tell these families that we're not making better progress underground," Murray said. "And it's strictly due to the fact that the mountain is still alive and the mountain is not allowing us to advance as rapidly as we would like to."

Rescuers plan to drill a fourth hole to try to find the miners, who've been trapped for 10 days in the central Utah coal mine. See photos of the rescue efforts »

Murray said it probably would take two days to drill the 1,586-foot-deep shaft.

On Wednesday, underground listening devices, called geophones, picked up a "series of spikes" over a five-minute period.

Rescuers said they didn't know what the sounds were, but they said they were encouraged.

Murray said the sensors have not picked up the noises again.

"Those sounds could have been anything, and we've said that from the beginning," Murray said Thursday.

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Two earlier bore holes have yielded no signs of life, but they have shown there is survivable space and some breathable air, rescuers have said.

During the drilling of the third bore hole Wednesday morning, two of the six geophone listening devices on the mountain above the mine detected the unknown noise, Stickler said. It lasted for about five minutes, stopped and has not been detected since, he said.

Geophones have never been successfully used to rescue trapped miners, but, in experiments, they have been able to pick up signals as far as 2,000 feet under the surface, Stickler said.

The six miners were trapped August 6, when the mine collapsed. In addition to drilling holes down into the mine, rescuers also have been working around the clock to clear rubble from a 2,000-foot stretch of tunnel leading to where the miners were thought to have been working. However, that work has been moving slowly, hampered by unstable conditions and seismic shifts inside the mine.

Murray maintains that an earthquake, not mining activity, caused the collapse. However, seismologists at the U.S. Geological Survey say there was no earthquake and the collapse registered as a 3.9-magnitude quake.

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